Genes and Variation

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Genes and Variation

• This group of ladybug beetles illustrates a
  population with a number of inherited traits
• Darwin recognized such variations as the raw
  material for evolution

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Genes and Variation
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Genes and Variation

• As Darwin developed his theory of evolution, he
  worked under a serious handicap
• He didn't know how heredity worked!
• Although Mendel's work on inheritance in peas was
  published during Darwin's lifetime, its
  importance wasn't recognized for decades
• This lack of knowledge left two big gaps in
  Darwin's thinking
• First, he had no idea how heritable traits pass
  from one generation to the next
• Second, although variation in heritable traits
  was central to Darwin's theory, he had no idea
  how that variation appeared

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Genes and Variation

• Evolutionary biologists connected Mendel's work
  to Darwin's during the 1930s
• By then, biologists understood that genes control
  heritable traits
• They soon realized that changes in genes produce
  heritable variation on which natural selection
  can operate
• Genes became the focus of new hypotheses and
  experiments aimed at understanding evolutionary
  change
• Another revolution in evolutionary thought began
  with Watson and Crick's studies on DNA
• Their model of the DNA molecule helped
  evolutionary biologists because it demonstrated
  the molecular nature of mutation and genetic
  variation

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Genes and Variation

• Today, molecular techniques are used to test
  hypotheses about how heritable variation appears
  and how natural selection operates on that
  variation
• Fitness, adaptation, species, and evolutionary
  change are now defined in genetic terms
• We understand how evolution works better than
  Darwin ever could, beginning with heritable
  variation

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How Common Is Genetic Variation?

• We now know that many genes have at least two
  forms, or alleles
• Animals such as horses, dogs, and mice often have
  several alleles for traits such as body size or
  coat color
• Plants, such as peas, often have several alleles
  for flower color
• All organisms have additional genetic variation
  that is invisible because it involves small
  differences in biochemical processes
• In addition, an individual organism is
  heterozygous for many genes
• An insect may be heterozygous for as many as 15
  percent of its genes
• Individual fishes, reptiles, and mammals are
  typically heterozygous for between 4 and 8
  percent of their genes

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Variation and Gene Pools

• Genetic variation is studied in populations
• A population is a group of individuals of the
  same species that interbreed
• Because members of a population interbreed, they
  share a common group of genes called a gene pool
• A gene pool consists of all genes, including all
  the different alleles, that are present in a
  population

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Variation and Gene Pools

• The relative frequency of an allele is the number
  of times that the allele occurs in a gene pool,
  compared with the number of times other alleles
  for the same gene occur
• Relative frequency is often expressed as a
  percentage
• Example
• In the mouse population in the figure at right,
  the relative frequency of the dominant B allele
  (black fur) is 40 percent, and the relative
  frequency of the recessive b allele (brown fur)
  is 60 percent
• The relative frequency of an allele has nothing
  to do with whether the allele is dominant or
  recessive
• In this particular mouse population, the
  recessive allele occurs more frequently than the
  dominant allele

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Variation and Gene Pools
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Variation and Gene Pools

• When scientists determine whether a population is
  evolving, they may look at the sum of the
  populations alleles, or its gene pool
• This diagram shows the gene pool for fur color in
  a population of mice
• Here, in a total of 50 alleles, 20 alleles are B
  (black), and 30 are b (brown)
• How many of each allele would be present in a
  total of 100 alleles?

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Variation and Gene Pools

• Gene pools are important to evolutionary theory,
  because evolution involves changes in populations
  over time
• In genetic terms, evolution is any change in the
  relative frequency of alleles in a population
• For example, if the relative frequency of the B
  allele in the mouse population changed over time
  to 30 percent, the population is evolving

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Sources of Genetic Variation

• Biologists can now explain how variation is
  produced
• The two main sources of genetic variation are
  mutations and the genetic shuffling that results
  from sexual reproduction

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Mutations

• A mutation is any change in a sequence of DNA
• Mutations can occur because of mistakes in the
  replication of DNA or as a result of radiation or
  chemicals in the environment
• Mutations do not always affect an organism's
  phenotypeits physical, behavioral, and
  biochemical characteristics
• For example, a DNA codon altered from GGA to GGU
  will still code for the same amino acid, glycine
• That mutation has no effect on phenotype
• Many mutations do produce changes in phenotype,
  however
• Some can affect an organism's fitness, or its
  ability to survive and reproduce in its
  environment
• Other mutations may have no effect on fitness

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Gene Shuffling 

• Mutations are not the only source of heritable
  variation
• You do not look exactly like your biological
  parents, even though they provided you with all
  your genes
• You probably look even less like any brothers or
  sisters you may have
• Yet, no matter how you feel about your relatives,
  mutant genes are not primarily what makes them so
  different from you

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Gene Shuffling 

• Most heritable differences are due to gene
  shuffling that occurs during the production of
  gametes
• Recall that each chromosome of a homologous pair
  moves independently during meiosis
• As a result, the 23 pairs of chromosomes found in
  humans can produce 8.4 million different
  combinations of genes!

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Gene Shuffling 

• Another process, crossing-over, also occurs
  during meiosis
• Crossing-over further increases the number of
  different genotypes that can appear in offspring
• Recall that a genotype is an organism's genetic
  makeup
• When alleles are recombined during sexual
  reproduction, they can produce dramatically
  different phenotypes
• Thus, sexual reproduction is a major source of
  variation within many populations

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Gene Shuffling 

• Sexual reproduction can produce many different
  phenotypes, but it does not change the relative
  frequency of alleles in a population
• To understand why, compare a population's gene
  pool to a deck of playing cards
• Each card represents an allele found in the
  population
• The exchange of genes during gene shuffling is
  similar to shuffling a deck of cards
• Shuffling leads to different types of hands, but
  it can never change the relative numbers of aces,
  kings, or queens in the deck
• The probability of drawing an ace off the top of
  the deck will always be 4 in 52, or one
  thirteenth (4/52 1/13)
• No matter how many times you shuffle the deck,
  this probability will remain the same
• Similarly, sexual reproduction produces many
  different combinations of genes, but in itself it
  does not alter the relative frequencies of each
  type of allele in a population

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GENE POOL

• The entire genetic content of a population is
  called the gene pool
• Contains all the genes for all the
  characteristics of a population
• Example all the marbles in the barrel represent
  the gene pool for coat color
• The fraction of marbles that represents a
  particular allele is called the gene frequency
  which may be expressed as a decimal or as a
  percent
• The sum of all the allele frequencies for a gene
  within a population is equal to 1.0 or 100
• In the following illustration 40 of the marbles
  are white and 60 of the marbles are brown (the
  frequencies can be expressed as 0.40 and 0.60
  respectfully)
• Dominant allele B (brown fur) (brown marble)
• Recessive allele b (white fur) (white marble)

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GENE POOL
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Single-Gene and Polygenic Traits

• Heritable variation can be expressed in a variety
  of ways
• The number of phenotypes produced for a given
  trait depends on how many genes control the trait
• Among humans, a widow's peaka downward dip in
  the center of the hairlineis a single-gene trait
• It is controlled by a single gene that has two
  alleles
• The allele for a widow's peak is dominant over
  the allele for a hairline with no peak
• As a result, variation in this gene leads to only
  two distinct phenotypes

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Single-Gene and Polygenic Traits
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Single-Gene and Polygenic Traits

• Single-Gene Traits
• In humans, a single gene with two alleles
  controls whether a person has a widow's peak or
  does not have a widow's peak
• As a result, only two phenotypes are possible
• The number of phenotypes a given trait has is
  determined by how many genes control the trait

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Single-Gene and Polygenic Traits

• As you can see, the frequency of phenotypes
  caused by this single gene is represented on the
  bar graph
• This graph shows that the presence of a widow's
  peak may be less common in a population than the
  absence of a widow's peak, even though the allele
  for a widow's peak is the dominant form
• In real populations, phenotypic ratios are
  determined by the frequency of alleles in the
  population as well as by whether the alleles are
  in the dominant or recessive form
• Allele frequencies may not match Mendelian ratios

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Single-Gene and Polygenic Traits

• Many traits are controlled by two or more genes
  and are, therefore, called polygenic traits
• Each gene of a polygenic trait often has two or
  more alleles
• As a result, one polygenic trait can have many
  possible genotypes and phenotypes

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Single-Gene and Polygenic Traits

• Height in humans is one example of a polygenic
  trait
• You can sample phenotypic variation in this trait
  by measuring the height of all the students in
  your class
• You can then calculate the average height of this
  group
• Many students will be just a little taller or
  shorter than average
• Some of your classmates, however, will be very
  tall or very short
• If you graph the number of individuals of each
  height, you may get a graph similar to the one
  shown below
• The symmetrical bell-like shape of this curve is
  typical of polygenic traits
• A bell-shaped curve is also called a normal
  distribution

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Single-Gene and Polygenic Traits

• Polygenic Trait
• The graph shows the distribution of phenotypes
  that would be expected for a trait if many genes
  contributed to the trait

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Evolution as Genetic Change

• A genetic view of evolution offers a new way to
  look at key evolutionary concepts
• Each time an organism reproduces, it passes
  copies of its genes to its offspring
• We can therefore view evolutionary fitness as an
  organism's success in passing genes to the next
  generation
• In the same way, we can view an evolutionary
  adaptation as any genetically controlled
  physiological, anatomical, or behavioral trait
  that increases an individual's ability to pass
  along its genes

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Evolution as Genetic Change

• Natural selection never acts directly on genes
• Why?
• Because it is an entire organismnot a single
  genethat either survives and reproduces or dies
  without reproducing
• Natural selection, therefore, can only affect
  which individuals survive and reproduce and which
  do not
• If an individual dies without reproducing, the
  individual does not contribute its alleles to the
  population's gene pool
• If an individual produces many offspring, its
  alleles stay in the gene pool and may increase in
  frequency

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Evolution as Genetic Change

• Now recall that evolution is any change over time
  in the relative frequencies of alleles in a
  population
• This reminds us that it is populations, not
  individual organisms, that can evolve over time

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Natural Selection on Single-Gene Traits

• Natural selection on single-gene traits can lead
  to changes in allele frequencies and thus to
  evolution
• Imagine that a hypothetical population of
  lizards, shown in the figure at right, is
  normally brown, but experiences mutations that
  produce red and black forms
• What happens to those new alleles?
• If red lizards are more visible to predators,
  they might be less likely to survive and
  reproduce, and the allele for red coloring might
  not become common

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Natural Selection on Single-Gene Traits
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Natural Selection on Single-Gene Traits

• Natural selection on single-gene traits can lead
  to changes in alleles frequencies and thus to
  evolution
• Organisms of one color, for example, may produce
  fewer offspring than organisms of other colors

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Natural Selection on Single-Gene Traits

• Black lizards, on the other hand, might absorb
  more sunlight and warm up faster on cold days
• If high body temperature allows them to move
  faster to feed and to avoid predators, they might
  produce more offspring than brown forms
• The allele for black color might then increase in
  relative frequency
• If a color change has no effect on fitness, the
  allele that produces it would not be under
  pressure from natural selection

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Natural Selection on Polygenic Traits

• When traits are controlled by more than one gene,
  the effects of natural selection are more complex
• As you learned earlier, the action of multiple
  alleles on traits such as height produces a range
  of phenotypes that often fit a bell curve
• The fitness of individuals close to one another
  on the curve will not be very different
• But fitness can vary a great deal from one end of
  such a curve to the other
• And where fitness varies, natural selection can
  act
• Natural selection can affect the distributions of
  phenotypes in any of three ways
• Directional selection
• Stabilizing selection
• Disruptive selection

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Directional Selection 

• When individuals at one end of the curve have
  higher fitness than individuals in the middle or
  at the other end, directional selection takes
  place
• The range of phenotypes shifts as some
  individuals fail to survive and reproduce while
  others succeed
• To understand this, consider how limited
  resources, such as food, can affect the long-term
  survival of individuals and the evolution of
  populations

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Directional Selection 

• Among seed-eating birds such as Darwin's finches,
  for example, birds with bigger, thicker beaks can
  feed more easily on larger, harder,
  thicker-shelled seeds
• Suppose a food shortage causes the supply of
  small and medium-sized seeds to run low, leaving
  only larger seeds
• Birds whose beaks enable them to open those
  larger seeds will have better access to food
• Birds with the big-beak adaptation would
  therefore have higher fitness than small-beaked
  birds
• The average beak size of the population would
  probably increase

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Directional Selection 
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Directional Selection 

• Directional Selection    Directional selection
  occurs when individuals at one end of the curve
  have higher fitness than individuals in the
  middle or at the other end. In this example, a
  population of seed-eating birds experiences
  directional selection when a food shortage causes
  the supply of small seeds to run low. The dotted
  line shows the original distribution of beak
  sizes. The solid line shows how the distribution
  of beak sizes would change as a result of
  selection.

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DIRECTIONAL NATURAL SELECTION

• Type of Natural Selection in which individuals
  with one of the extreme forms of a trait have an
  advantage in terms of survival and reproduction

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DIRECTIONAL NATURAL SELECTION
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Stabilizing Selection 

• When individuals near the center of the curve
  have higher fitness than individuals at either
  end of the curve, stabilizing selection takes
  place
• This situation keeps the center of the curve at
  its current position, but it narrows the overall
  graph

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Stabilizing Selection 

• The mass of human infants at birth is under the
  influence of stabilizing selection
• Human babies born much smaller than average are
  likely to be less healthy and thus less likely to
  survive
• Babies that are much larger than average are
  likely to have difficulty being born
• The fitness of these larger or smaller
  individuals is, therefore, lower than that of
  more average-sized individuals

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Stabilizing Selection 
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Stabilizing Selection 

• Stabilizing selection takes place when
  individuals near the center of a curve have
  higher fitness than individuals at either end
• This example shows that human babies born at an
  average mass are more likely to survive than
  babies born either much smaller or much larger
  than average

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STABILIZING NATURAL SELECTION

• Type of Natural Selection in which individuals
  with the average form of a trait have an
  advantage in terms of survival and reproduction
• Extreme forms of the trait confers a disadvantage
  to the organism

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STABILIZING NATURAL SELECTION
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Disruptive Selection 

• When individuals at the upper and lower ends of
  the curve have higher fitness than individuals
  near the middle, disruptive selection takes place
• In such situations, selection acts most strongly
  against individuals of an intermediate type
• If the pressure of natural selection is strong
  enough and lasts long enough, this situation can
  cause the single curve to split into two
• In other words, selection creates two distinct
  phenotypes

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Disruptive Selection 

• For example, suppose a population of birds lives
  in an area where medium-sized seeds become less
  common and large and small seeds become more
  common
• Birds with unusually small or large beaks would
  have higher fitness
• The population might split into two subgroups
• One that eats small seeds
• One that eats large seeds

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Disruptive Selection 
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Disruptive Selection 

• When individuals at the upper and lower ends of
  the curve have higher fitness than individuals
  near the middle, disruptive selection takes place
• In this example, average-sized seeds become less
  common, and larger and smaller seeds become more
  common
• As a result, the bird population splits into two
  subgroups specializing in eating different-sized
  seeds.

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DISRUPTIVE NATURAL SELECTION

• Type of Natural Selection in which individuals
  with either of the extreme forms of a trait have
  an advantage in terms of survival and reproduction

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DISRUPTIVE NATURAL SELECTION
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Genetic Drift

• Natural selection is not the only source of
  evolutionary change
• In small populations, an allele can become more
  or less common simply by chance
• Recall that genetics is controlled by the laws of
  probability
• These laws can be used to predict the overall
  results of genetic crosses in large populations
• However, the smaller a population is, the farther
  the results may be from what the laws of
  probability predict
• This kind of random change in allele frequency is
  called genetic drift
• How does genetic drift take place?
• In small populations, individuals that carry a
  particular allele may leave more descendants than
  other individuals do, just by chance
• Over time, a series of chance occurrences of this
  type can cause an allele to become common in a
  population

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Genetic Drift

• Genetic drift may occur when a small group of
  individuals colonizes a new habitat
• These individuals may carry alleles in different
  relative frequencies than did the larger
  population from which they came
• If so, the population that they found will be
  genetically different from the parent population
• Here, however, the cause is not natural selection
  but simply chancespecifically, the chance that
  particular alleles were in one or more of the
  founding individuals
• A situation in which allele frequencies change as
  a result of the migration of a small subgroup of
  a population is known as the founder effect
• One example of the founder effect is the
  evolution of several hundred species of fruit
  flies found on different Hawaiian Islands
• All of those species descended from the same
  original mainland population
• Those species in different habitats on different
  islands now have allele frequencies that are
  different from those of the original species

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Genetic Drift

• In small populations, individuals that carry a
  particular allele may have more descendants than
  other individuals
• Over time, a series of chance occurrences of this
  type can cause an allele to become more common in
  a population
• This model demonstrates how two small groups from
  a large, diverse population could produce new
  populations that differ from the original group

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Genetic Drift
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Evolution Versus Genetic Equilibrium

• To clarify how evolutionary change operates,
  scientists often find it helpful to determine
  what happens when no change takes place
• So biologists ask Are there any conditions under
  which evolution will not occur?
• Is there any way to recognize when that is the
  case?
• The answers to those questions are provided by
  the Hardy-Weinberg principle, named after two
  researchers who independently proposed it in 1908

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Hardy-Weinberg Principle

• The Hardy-Weinberg principle states that allele
  frequencies in a population will remain constant
  unless one or more factors cause those
  frequencies to change
• The situation in which allele frequencies remain
  constant is called genetic equilibrium
• If the allele frequencies do not change, the
  population will not evolve

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GENE POOL

• Hardy-Weinberg Principle
• Demonstrates how the frequency of alleles in the
  gene pool can be described by mathematical
  formulas
• Shows that under certain conditions the frequency
  of genes remains constant from generation to
  generation
• States that the frequency of dominant and
  recessive alleles remains the same from
  generation to generation

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HARDY-WEINBERG PRINCIPLE

• Useful in population genetics
• 1 p2 2(pq) q2 (1 B2 2(Bb) b2)
• Previous example
• When rabbits mate and produce offspring, each
  parent contributes one allele for coat color to
  each gamete (randomly reach in the barrel and
  remove one marble)
• Offspring are produced when two gametes fuse to
  form a zygote (represented by a pair of marbles
  each randomly removed individually)
• Chance of removing a particular color marble
  depends on the frequency of different marbles in
  the gene pool
• The probability of drawing a particular genotype
  is the product of the probabilities of the two
  alleles
• Probabilities can be demonstrated with a
  Cross-Multiplication Table

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CROSS-MULTIPLICATION TABLE
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HARDY-WEINBERG PRINCIPLE

• As long as any color rabbit is allowed to mate
  with any other color rabbit, the probability of
  drawing each genotype will remain constant
• After 15 or even after 40 generations, there will
  be 84 brown rabbits and 16 white rabbits
• Recessive genes will not be lost in a population
  over time
• Some diseases are homozygous recessive and their
  frequencies in the population can be calculated
• Phenylketonuria (PKU) autosomal recessive
  disease caused by an error in human metabolism
• Results from the inability to break down
  phenylalanine, an amino acid that is common in
  many foods
• Most people produce an enzyme that converts
  phenylalanine to another amino acid
• Production of this enzyme is governed by a
  dominant allele (recessive allele does not
  produce this enzyme)
• Without the enzyme, phenylalanine builds up
  poisoning the brain and causing severe
  retardation
• Babies appear normal at birth
• Damages begins when the baby drinks milk which
  contains phenylalanine
• Most USA hospitals tests for PKU and if found a
  special diet must be followed for the first few
  years of life while the brain is developing
  (after a few years a normal diet can be resumed)
• Babies with PKU are born once in every 10,000
  births in USA (homozygous phenotype frequency is
  1/10,000 0.0001 or 0.01) (gene frequency is 1)

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HARDY-WEINBERG PRINCIPLE

• States that under certain conditions, gene
  frequencies will remain constant from generation
  to generation
• A population in which there is no change in gene
  frequency over a long period is said to be in
  genetic equilibrium
• In order to maintain genetic equilibrium five
  assumptions are necessary
• 1. No mutations occur
• 2. The population is large
• 3. Mating between males and females is random
• 4. Individuals do not leave the population or
  enter from outside
• 5.No phenotype is more likely to survive and have
  offspring than any other phenotype
• In natural populations, these conditions are
  rarely met
• The Hardy-Weinberg Principle is used to compare
  natural populations with an ideal situation
• When gene frequencies change from one generation
  to the next, the change is usually caused by a
  departure from one of these five assumptions

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Hardy-Weinberg Principle

• Under what conditions does the Hardy-Weinberg
  principle hold?
• Five conditions are required to maintain genetic
  equilibrium from generation to generation
• (1) There must be random mating
• (2) The population must be very large
• (3) There can be no movement into or out of the
  population
• (4) No mutations
• (5) No natural selection

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Hardy-Weinberg Principle

• In some populations, these conditions may be met
  or nearly met for long periods of time
• If, however, the conditions are not met, the
  genetic equilibrium will be disrupted, and the
  population will evolve

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Random Mating 

• All members of the population must have an equal
  opportunity to produce offspring
• Random mating ensures that each individual has an
  equal chance of passing on its alleles to
  offspring

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Random Mating 

• In natural populations, however, mating is rarely
  completely random
• Many species, including lions and wolves, select
  mates based on particular heritable traits, such
  as size or strength
• Such nonrandom mating means that the genes for
  those traits are not in equilibrium but are under
  strong selection pressure

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HARDY-WEINBERG PRINCIPLE

• Assumption Mating between males and females is
  random
• The Effect of Nonrandom Mating
• Assortative Mating Some organisms are more
  likely to mate with similar organisms than with
  dissimilar organisms (same nationality)
• Frequency of recessive alleles will appear to be
  higher
• Does not alter the gene frequency in a
  population, but does change the frequency of
  phenotypes

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Large Population 

• A large population size is also important in
  maintaining genetic equilibrium
• Genetic drift has less effect on large
  populations than on small ones
• That is because the allele frequencies of large
  populations are less likely to be changed through
  the process of genetic drift

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HARDY-WEINBERG PRINCIPLE

• Assumption The population is large
• The Effect of Small Population
• Flipping of a coin is a 50-50 chance of heads or
  tails
• But in a small sampling you might get a higher
  percentage of one or the other
• In a small population a rare allele may be lost
  or may become unusually common
• Genetic drift a change in gene frequency due to
  random variations in a small population
• Amish community tend to marry among themselves
• High frequency of a severe enzyme-deficiency
  disease

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HARDY-WEINBERG PRINCIPLE
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HARDY-WEINBERG PRINCIPLE
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No Movement Into or Out of the Population 

• Because individuals may bring new alleles into a
  population, there must be no movement of
  individuals into or out of a population
• In genetic terms, the population's gene pool must
  be kept together and kept separate from the gene
  pools of other populations

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HARDY-WEINBERG PRINCIPLE

• Assumption Individuals do not leave the
  population or enter from outside
• The Effect of Migration
• Genetic equilibrium will be altered if organisms
  can move in or out of a particular breeding
  population (migration)
• North America population of today very different
  from the original population of American Indians

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HARDY-WEINBERG PRINCIPLE
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No Mutations 

• If genes mutate from one form into another, new
  alleles may be introduced into the population,
  and allele frequencies will change

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HARDY-WEINBERG PRINCIPLE

• Assumption No mutations occur (ideal)
• The Effect of Mutations (reality)
• Mutations are the original source of variations
  in populations
• All genes are subject to mutations
• Mutations change the frequency of alleles in a
  population
• Example
• mutations continually add genes for hemophilia
  to the human gene pool
• Mutations for hemophilia gene occur about 3 times
  in every 100,000 gametes
• Are we weakening our gene pool?

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HARDY-WEINBERG PRINCIPLE
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No Natural Selection 

• All genotypes in the population must have equal
  probabilities of survival and reproduction
• No phenotype can have a selective advantage over
  another
• In other words, there can be no natural selection
  operating on the population

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HARDY-WEINBERG PRINCIPLE

• Assumption No phenotype is more likely to
  survive and have offspring than any other
  phenotype
• No genotype is more advantageous to an individual
  than any other genotype
• The Effect of Harmful Genes
• Organisms that are homozygous for harmful genes
  are less likely to survive and produce offspring
  than those that do not carry such genes
• Over many generations harmful genes will become
  less frequent in the population
• True Today ????? Are we weakening the gene pool
  ?????
• The Hardy-Weinberg Principle states that for a
  population to remain in genetic equilibrium
  natural selection must not occur
• But in naturally occurring populations, one
  allele is often more advantageous to an organism
  than another allele (natural selection does
  occur)
• This violation of the Hardy-Weinberg assumptions
  is so common that it is the basis of EVOLUTION

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Should the Use of Antibiotics Be Restricted?

• Natural selection is everywhere
• One dramatic example of evolution in action poses
  a serious threat to public health
• Many kinds of disease-causing bacteria are
  evolving resistance to antibioticsdrugs intended
  to kill them or interfere with their growth

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Should the Use of Antibiotics Be Restricted?

• Antibiotics are one of medicine's greatest
  weapons against bacterial diseases
• When antibiotics were discovered, they were
  called magic bullets and wonder drugs because
  they were so effective
• They have made diseases like pneumonia much less
  of a threat than they were about sixty years ago
• However, people may be overusing antibiotics
• Doctors sometimes prescribe them for diseases for
  which they are not effective
• Commercial feed for chickens and other farm
  animals is laced with antibiotics to prevent
  infection

84
Should the Use of Antibiotics Be Restricted?

• This wide use has caused many bacteriaincluding
  Mycobacterium tuberculosis, which causes
  tuberculosisto evolve resistance to antibiotics
• This resistance is a prime example of the
  evolution of a genetically controlled
  physiological trait
• Resistance evolved because bacterial populations
  contained a few individuals with genes that
  enabled them to destroy, inactivate, or eliminate
  antibiotics
• Descendants of those physiologically similar
  individuals survived and reproduced, and became
  today's resistant strains
• Once-powerful antibiotics are now useless against
  resistant bacteria
• Given this risk, should government agencies
  restrict the use of antibiotics?

85
The Viewpoints

• Antibiotic Use Should Be Restricted
• The danger of an incurable bacterial epidemic is
  so high that action must be taken on a national
  level as soon as possible
• Doctors overuse antibiotics in humans because
  patients demand them
• The livestock industry likes using antibiotics in
  animal feeds and will not change their practice
  unless forced to do so

86
The Viewpoints

• Antibiotic Use Should Not Be Restricted
• Researchers are coming up with new drugs all the
  time
• These drugs can be reserved for human use only
• Doctors need to be able to prescribe antibiotics
  as they choose, and our food supply depends on
  the use of antibiotics in agriculture
• The medical profession and the livestock industry
  need the freedom to find solutions that work best
  for them

87
NATURAL SELECTION

• Process by which organisms with favorable
  variations survive and reproduce at higher rates
  than those without such variations
• Ongoing process
• Single most significant factor disrupting genetic
  equilibrium
• Results in higher reproductive rates for
  individuals with certain phenotypes, and, hence,
  certain genotypes
• Some members of a population are more likely to
  contribute their genes to the next generation
  than others are
• Allele frequencies change from one generation to
  the next
• Four types of Natural Selection Stabilizing,
  Directional, Disruptive, Sexual
• Cause changes in the gene pool

88
The Process of Speciation

• Factors such as natural selection and chance
  events can change the relative frequencies of
  alleles in a population
• But how do these changes lead to the formation of
  new species, or speciation?

89
The Process of Speciation

• Recall that biologists define a species as a
  group of organisms that breed with one another
  and produce fertile offspring
• This means that individuals in the same species
  share a common gene pool
• Because a population of individuals has a shared
  gene pool, a genetic change that occurs in one
  individual can spread through the population as
  that individual and its offspring reproduce
• If a genetic change increases fitness, that
  allele will eventually be found in many
  individuals of that population

90
Isolating Mechanisms

• Given this genetic definition of species, what
  must happen for a species to evolve into two new
  species?
• The gene pools of two populations must become
  separated for them to become new species
• As new species evolve, populations become
  reproductively isolated from each other
• When the members of two populations cannot
  interbreed and produce fertile offspring,
  reproductive isolation has occurred
• At that point, the populations have separate gene
  pools
• They respond to natural selection or genetic
  drift as separate units
• Reproductive isolation can develop in a variety
  of ways, including
• Behavioral isolation
• Geographic isolation
• Temporal isolation

91
Behavioral Isolation 

• One type of isolating mechanism, behavioral
  isolation, occurs when two populations are
  capable of interbreeding but have differences in
  courtship rituals or other reproductive
  strategies that involve behavior
• For example, the eastern and western meadowlarks
  shown to the right are very similar birds whose
  habitats overlap in the center of the United
  States
• Members of the two species will not mate with
  each other, however, partly because they use
  different songs to attract mates
• Eastern meadowlarks will not respond to western
  meadowlark songs, and vice versa

92
Behavioral Isolation 
93
Behavioral Isolation 

• Behavioral Isolation
• The eastern meadowlark (left) and western
  meadowlark (right) have overlapping ranges
• They do not interbreed, however, because they
  have different mating songs

94
SEXUAL NATURAL SELECTION

• Preferential choice of a mate based on the
  presence of a specific trait

95
SPECIATION

• Reproductive Isolation
• Inability of formerly interbreeding organisms to
  produce offspring
• May result from disruptive natural selection of
  breeding times (early spring / early summer times
  favored while middle times fell prey to
  predation)
• A population that breeds in May is effectively
  isolated from one that breeds in July
• Eventually different selection pressures led to
  different mating times and different
  morphological variations (frogs of different
  colors)

96
REPRODUCTIVE ISOLATION
97
REPRODUCTIVE ISOLATION
98
REPRODUCTIVE ISOLATION
99
REPRODUCTIVE ISOLATION
100
REPRODUCTIVE ISOLATION
101
SPECIATION

• Formation of a new species
• Often occurs when part of a population becomes
  isolated from the rest of the population
• Since no two environments are identical,
  selective pressures that occur in one location
  may be different from the pressures in another
  location

102
SPECIATION

• Geographic Isolation
• Occurs when a physical barrier develops between a
  segment of two populations
• Squirrels at Grand Canyon
• Honeycreepers (finches) of the Hawaiian Islands
• Finches / Tortoises of the Galapagos Islands
• Death Valley fish

103
Geographic Isolation 

• With geographic isolation, two populations are
  separated by geographic barriers such as rivers,
  mountains, or bodies of water
• The Abert squirrel, for example, lives in the
  Southwest
• About 10,000 years ago, the Colorado River split
  the species into two separate populations
• Two separate gene pools formed
• Genetic changes that appeared in one group were
  not passed to the other
• Natural selection worked separately on each group
  and led to the formation of a distinct
  subspecies
• The Abert and Kaibab squirrels have very similar
  anatomical and physiological characteristics,
  indicating that they are closely related
• However, the Kaibab squirrel differs from the
  Abert squirrel in significant ways, such as fur
  coloring

104
Geographic Isolation 
105
Geographic Isolation 

• When two populations of a species become
  reproductively isolated, new species can develop
• The Kaibab squirrel evolved from the Abert
  squirrel
• The Kaibab squirrels were isolated from the main
  population by the Colorado River

106
GEOGRAPHIC ISOLATION
107
GEOGRAPHIC ISOLATION
108
GEOGRAPHIC ISOLATION
109
Geographic Isolation 

• Geographic barriers do not guarantee the
  formation of new species, however
• Separate lakes may be linked for a time during a
  flood, or a land bridge may temporarily form
  between islands, enabling separated populations
  to mix
• If two formerly separated populations can still
  interbreed, they remain a single species
• Also, any potential geographic barrier may
  separate certain types of organisms but not
  others
• A large river will keep squirrels and other small
  rodents apart, but it does not necessarily
  isolate bird populations

110
Temporal Isolation 

• A third isolating mechanism is temporal
  isolation, in which two or more species reproduce
  at different times
• For example, three similar species of orchid all
  live in the same rain forest
• Each species releases pollen only on a single day
• Because the three species release pollen on
  different days, they cannot pollinate one another

111
Testing Natural Selection in Nature

• Now that you know the basic mechanisms of
  evolutionary change, you might wonder if these
  processes can be observed in nature
• The answer is yes
• In fact, some of the most important studies
  showing natural selection in action involve
  descendants of the finches that Darwin observed
  in the Galápagos Islands

112
Testing Natural Selection in Nature

• Those finch species looked so different from one
  another that when Darwin first saw them, he did
  not realize they were all finches
• He thought they were blackbirds, warblers, and
  other kinds of birds
• The species he examined differed greatly in the
  sizes and shapes of their beaks and in their
  feeding habits
• Some species fed on small seeds, while others ate
  large seeds with thick shells
• One species used cactus spines to pry insects
  from dead wood
• One species, not shown here, even pecked at the
  tails of large sea birds and drank their blood!

113
Testing Natural Selection in Nature

• Detailed genetic studies have shown that these
  finches evolved from a species with a
  more-or-less general-purpose beak

114
Testing Natural Selection in Nature
115
Testing Natural Selection in Nature

• Once Darwin discovered that these birds were all
  finches, he hypothesized that they had descended
  from a common ancestor
• Over time, he proposed, natural selection shaped
  the beaks of different bird populations as they
  adapted to eat different foods

116
Testing Natural Selection in Nature

• That was a reasonable hypothesis
• But was there any way to test it?
• No one thought so, until the work of Peter and
  Rosemary Grant from Princeton University proved
  otherwise
• For more than twenty years, the Grants have been
  collaborating to band and measure finches on the
  Galápagos Islands
• They realized that Darwin's hypothesis relied on
  two testable assumptions
• First, in order for beak size and shape to
  evolve, there must be enough heritable variation
  in those traits to provide raw material for
  natural selection
• Second, differences in beak size and shape must
  produce differences in fitness that cause natural
  selection to occur

117
Testing Natural Selection in Nature

• The Grants tested these hypotheses on the medium
  ground finch on Daphne Major, one of the
  Galápagos Islands
• This island is large enough to support good-sized
  finch populations, yet small enough to enable the
  Grants to catch and identify nearly every bird
  belonging to the species under study

118
Variation 

• The Grants first identified and measured as many
  individual birds as possible on the island
• They recorded which birds were still living and
  which had died, which had succeeded in breeding
  and which had not
• For each individual, they also recorded
  anatomical characteristics such as wing length,
  leg length, beak length, beak depth, beak color,
  feather colors, and total mass
• Many of these characteristics appeared in
  bell-shaped distributions typical of polygenic
  traits
• These data indicate that there is great variation
  of heritable traits among the Galápagos finches

119
Natural Selection 

• Other researchers who had visited the Galápagos
  did not see the different finches competing or
  eating different foods
• During the rainy season, when these researchers
  visited, there is plenty of food
• Under these conditions, finches often eat the
  most available type of food
• During dry-season drought, however, some foods
  become scarce, and others disappear altogether
• At that time, differences in beak size can mean
  the difference between life and death
• To survive, birds become feeding specialists
• Each species selects the type of food its beak
  handles best
• Birds with big, heavy beaks, for example, select
  big, thick seeds that no other species can crack
  open

120
Natural Selection 

• The Grants' most interesting discovery was that
  individual birds with different-sized beaks had
  different chances of survival during a drought
• When food for the finches was scarce, individuals
  with the largest beaks were more likely to
  survive
• Beak size also plays a role in mating behavior,
  because big-beaked birds tend to mate with other
  big-beaked birds
• The Grants observed that average beak size in
  that finch population increased dramatically over
  time
• This change in beak size is an example of
  directional selection operating on an anatomical
  trait

121
Natural Selection
122
Natural Selection

• Survival Rate
• This graph shows the survival rate of one species
  of ground-feeding finches, the medium ground
  finch, Geospiza fortis.

123
Rapid Evolution 

• By documenting natural selection in the wild, the
  Grants provided evidence of the process of
  evolution
• The next generation of finches had larger beaks
  than did the generation before selection had
  occurred
• An important result of this work was their
  finding that natural selection takes place
  frequentlyand sometimes very rapidly
• Changes in the food supply on the Galápagos
  caused measurable fluctuations in the finch
  populations over a period of only decades
• This is markedly different from the slow, gradual
  evolution that Darwin envisioned

124
Speciation in Darwin's Finches

• The Grants' work demonstrates that finch beak
  size can be changed by natural selection
• If we combine this information with other
  evolutionary concepts you have learned in this
  chapter, we can show how natural selection can
  lead to speciation
• We can devise a hypothetical scenario for the
  evolution of all Galápagos finches from a single
  group of founding birds
• Speciation in the Galápagos finches occurred by
  founding of a new population, geographic
  isolation, changes in the new population's gene
  pool, reproductive isolation, and ecological
  competition

125
Founders Arrive 

• Many years ago, a few finches from the South
  American mainlandspecies Aflew or were blown to
  one of the Galápagos Islands, as shown the
  activity at right
• Finches are small birds that do not usually fly
  far over open water
• These birds may have gotten lost, or they may
  have been blown off course by a storm
• Once they arrived on one of the islands, they
  managed to survive and reproduce

126
Geographic Isolation 

• Later on, some birds from species A crossed to
  another island in the Galápagos group
• Because these birds do not usually fly over open
  water, they rarely move from island to island
• Thus, finch populations on the two islands were
  essentially isolated from each other and no
  longer shared a common gene pool

127
Changes in the Gene Pool 

• Over time, populations on each island became
  adapted to their local environments
• The plants growing on the first island may have
  produced small thin-shelled seeds, whereas the
  plants on the second island may have produced
  larger thick-shelled seeds
• On the second island, directional selection would
  favor individuals with larger, heavier beaks
• These birds could crack open and eat the large
  seeds more easily
• Thus, birds with large beaks would be better able
  to survive on the second island
• Over time, natural selection would have caused
  that population to evolve larger beaks, forming a
  separate population, B

128
Reproductive Isolation 

• Now, imagine that a few birds from the second
  island cross back to the first island
• Will the population-A birds breed with the
  population-B birds? Probably not
• These finches choose their mates carefully
• As part of their courtship behavior, they inspect
  a potential partner's beak very closely
• Finches prefer to mate with birds that have the
  same-sized beak as they do
• In other words, big-beaked birds prefer to mate
  with other big-beaked birds, and smaller-beaked
  birds prefer to mate with other smaller-beaked
  birds
• Because the birds on the two islands have
  different-sized beaks, it is likely that they
  would not choose to mate with each other
• Thus, differences in beak size, combined with
  mating behavior, could lead to reproductive
  isolation
• The gene pools of the two bird populations remain
  isolated from each othereven when individuals
  live together in the same place
• The two populations have now become separate
  species

129
Ecological Competition 

• As these two new species live together in the
  same environment (the first island), they compete
  with each other for available seeds
• During the dry season, individuals that are most
  different from each other have the highest
  fitness
• The more specialized birds have less competition
  for certain kinds of seeds and other foods, and
  the competition among individual finches is also
  reduced
• Over time, species evolve in a way that increases
  the differences between them
• The species-B birds on the first island may
  evolve into a new species, C

130
Continued Evolution 

• This process of isolation on different islands,
  genetic change, and reproductive isolation
  probably repeated itself time and time again
  across the entire Galápagos island chain
• Over many generations, it produced the 13
  different finch species found there today
• Use the steps in the activity Speciation in
  Galápagos Finches to explain how other Darwin
  finches, such as the vegetarian tree finch that
  feeds on fruit, might have evolved

131
Studying Evolution Since Darwin

• It is useful to review and critique the strengths
  and weaknesses of evolutionary theory
• Darwin made bold assumptions about heritable
  variation, the age of Earth, and relationships
  among organisms
• New data from genetics, physics, and biochemistry
  could have proved him wrong on many counts
• They didn't
• Scientific evidence supports the theory that
  living species descended with modification from
  common ancestors that lived in the ancient past

132
Limitations of Research 

• The Grants' research clearly shows the effects of
  directional selection in nature
• The Grants' data also show how competition and
  climate change affect natural selection
• The work does have limitations
• For example, while the Grants observed changes in
  the size of the finches' beaks, they did not
  observe the formation of a new species
• Scientists predict that as new fossils are found,
  they will continue to expand our understanding of
  how species evolved

133
Unanswered Questions 

• The studies of the Grants fit into an enormous
  body of scientific work supporting the theory of
  evolution
• Millions of fossils show that life has existed on
  Earth for more than 3 billion years and that
  organisms have changed dramatically over this
  time
• These fossils form just a part of the evidence
  supporting the conclusion that life has evolved
• Remember that a scientific theory is defined as a
  well-tested explanation that accounts for a broad
  range of observations
• Evolutionary theory fits this definition
• To be sure, many new discoveries have led to new
  hypotheses that refine and expand Darwin's
  original ideas
• No scientist suggests that all evolutionary
  processes are fully understood
• Many unanswered questions remain

134
Unanswered Questions 

• Why is understanding evolution important?
• Because evolution continues today, driving
  changes in the living world such as drug
  resistance in bacteria and viruses, and pesticide
  resistance in insects
• Evolutionary theory helps us understand and
  respond to these changes in ways that improve
  human life

135
RATES OF SPECIATION

• Sometimes requires millions of years but some
  species can form more rapidly
• Divergence of organisms and thus speciation may
  not occur smoothly and gradually but in spurts
• Fossil record suggests that rapid speciation may
  be the norm rather than the exception
• Punctuated Equilibrium
• Indicates that many species existed without
  change for a long periods of time (close to
  genetic equilibrium)
• The periods of stability were separated by an
  instant change in terms of geological time (a
  few thousand rather than a few million years)
• Punctuated part of this term refers to the sudden
  shift in form that is often seen in the fossil
  record
• Equilibrium may be interrupted by a brief period
  of rapid genetic change in which speciation
  occurs
• If it was gradual, there should be intermediate
  forms (none in the fossil record)

136
RATES OF SPECIATION
137
EXTINCTION

• Just as new species form through natural
  selection, species also die off (become extinct)
• Changes in climate and competition has an effect
• Destruction of habitats
• Natural process but humans have accelerated it